In recent years, demand for components having high abrasion resistance and shock resistance is increasing in mobile electronic devices typified by smartphones. Particularly, since exterior components of mobile electronic devices have a thin plate shape with a thickness of about 1 to about 2 mm and must withstand impact such as falling impact, a particularly high-shock resistant material is required for these exterior components.
A currently used material is tempered glass strengthened by ion exchange. A tempered layer of several tens of micrometers is formed on the surface of the glass by ion exchange to generate compressive stress on the surface, whereby propagation of cracks is prevented. However, since the strengthening mechanism of the tempered glass is due to the tempered layer, the tempered glass is broken immediately when a crack propagates across the tempered layer. The Vickers hardness of the glass is about 600. Therefore, a problem with the glass is that it is easily flawed by contact with metal, concrete, etc. and the strength is reduced significantly by flaws formed during use.
Ceramics are excellent in heat resistance, wear resistance, and corrosion resistance and are therefore widely used for industrial component applications. Particularly, zirconia sintered bodies have high strength, high toughness, high hardness, and abrasion resistance, and their design can be easily improved by coloration. Therefore, the zirconia sintered bodies are being increasingly used for watch components etc. The use of zirconia sintered bodies for exterior components of mobile electronic devices etc. is also contemplated. Particularly, when a zirconia sintered body is used for an exterior component of a mobile electronic device, the thickness of the zirconia sintered body must be increased in order to improve shock resistance. Therefore, the weight of the component becomes high, which is not practically advantageous. When the thickness is reduced to reduce the weight, resistance to impact of falling, collision, etc. becomes insufficient, and the zirconia sintered body is easily cracked and becomes unusable.
To improve the shock resistance of a ceramic component, a method similar to laminated glass has been proposed (for example, Patent Literatures 1 and 2). Specifically, the ceramic component is joined to a fiber-reinforced plastic to prevent a flying object such as a cannonball from piercing the ceramic component. Unfortunately, this method is for the purpose of preventing the penetration of the flying object, and cracking of the ceramic due to collision cannot be prevented.
Patent Literature 3 describes a watch cover glass produced by joining sapphire and inorganic glass. However, the purpose of this cover glass is to improve its abrasion resistance by disposing high-hardness sapphire on the surface of the watch cover glass, and high shock resistance required for the mobile electronic device applications cannot be obtained using this method.
Therefore, there has been no shock resistant component that is used for a zirconia sintered body plate having a thickness of several millimeters and has improved resistance to cracking due to impact of falling, collision, etc., and there has been no method of producing such a shock resistant component.